Cross-linked carboxylic group-containing polymers have been conventionally used as thickeners for various aqueous solutions. As these cross-linked carboxylic group-containing polymers, there have been known, for instance, a copolymer of an α,β-unsaturated carboxylic acid such as acrylic acid with a polyallyl ether (U.S. Pat. No. 2,798,053, U.S. Pat. No. 2,923,692, U.S. Pat. No. 4,267,103, U.S. Pat. No. 4,923,940); a copolymer of an α,β-unsaturated carboxylic acid with hexaallyl trimethylene trisulfone (U.S. Pat. No. 2,958,679); a copolymer of an α,β-unsaturated carboxylic acid with triallyl phosphate (U.S. Pat. No. 3,426,004); a copolymer of an α,β-unsaturated carboxylic acid with glycidyl methacrylate (Japanese Patent Laid-open No. Sho 58-84819) and the like.